A. Field of the Invention
The present invention relates generally to digital wireless communication systems, and, more particularly, to a digital wireless communications system that eliminates intersymbol interference (xe2x80x9cISIxe2x80x9d) and mutlipath cancellation using a plurality of optimal, ambiguity resistant (xe2x80x9cARxe2x80x9d) precoders.
B. Description of the Related Art
Digital data signals sent from a transmitter to a receiver over a data channel are often corrupted by the inherent characteristics of the channel. For example, if the data channel has a limited bandwidth, the pulses will start to overlap, which often leads to the situation where a signal received at a specific moment not only depends upon a single data symbol, but also on data symbols adjacent in time. This is known as intersymbol interference or ISI. Intersymbol interference may be caused not only by the limited bandwidth of the channel, but also by the use of a band limiting filter at the transmitter for giving the frequency spectrum of the transmitted and recorded pulses a desired shape. The presence of intersymbol interference will thus often lead to an increased symbol error rate.
In attempting to correct for intersymbol interference, it is common to supply an equalizer in the receiver. An equalizer is a digital hardware/software apparatus which corrects for the intersymbol interference of the received digitally-encoded signals so that the initial data can be recovered. Typically, the equalizer compensates for intersymbol interference via several processes, such as linear equalization or decision feedback equalization. In linear equalization, the incoming signals are multiplied by the inverse of the intersymbol interference, generally removing intersymbol interference from the incoming signals. A drawback of linear equalization is that noise inherent in the data transmission is undesirably simultaneously amplified. Decision feedback equalization avoids the noise amplification problems of linear equalization, but runs the risk of error propagation since any decision errors that are made are fed back.
Wireless communication is well known in the art. A xe2x80x9cwireless communications systemxe2x80x9d refers to a communications system having a transmitting end and a receiving end in which signals are transmitted or communicated from the transmitting end to the receiving end via a signal path, wherein a portion of the signal path from the transmitting end to the receiving end includes signal transmission via a wireless medium.
In digital wireless communications systems, antenna array receivers play an important role in reducing errors, such as intersymbol interference, in the digital signals received. One of the problems of a digital wireless communication system is the equalizing of the digitally-encoded signals, as noted above. As the digitally-encoded signal is transmitted from one unit to another, through a multiplicity of data paths, the various signals arriving at the other unit can cause delay spread between the digitally-encoded signals, also known as intersymbol interference.
The problem of intersymbol interference is especially true for low-power, high-speed, digital wireless communications systems, such as personal communication services (PCS). PCS is a new type of service being introduced in which subscribers are provided with a single, personal telephone number used to access those subscribers regardless of their physical location. For example, callers to a personal number may automatically be connected to a business, residence, cellular, or other phone, or voice messaging system, depending on where the individual they are trying to reach happens to be at the time of the call.
In PCS systems, however, high speed sampling at the receiver causes significant hardware cost. A proposed solution for reducing hardware costs involves providing a downsampling block in the receiver which reduces the sampling rate of the signals received at the receiver. In such sampled systems, the bandwidth of the received signals is reduced by downsampling. This involves dropping samples to reduce the number of signals received. A drawback of downsampling is that it causes information to be lost.
Another approach, as suggested by M. Tomlinson in xe2x80x9cNew Automatic Equalizer Employing Modulo Arithmetic,xe2x80x9d Electronics Letters, vol. 7, pp. 138-39 (March, 1971), involves eliminating intersymbol interference in the transmitter rather than in the receiver of the communications system. In other words, the signals are predistorted in the transmitter in such a manner that would cancel out the intersymbol interference of the channel upon transmission. The Tomlinson scheme precodes the signal data according to a linear function and is generally effective. However, Tomlinson precoding substantially destroys a desired power distribution of coded signals.
Intersymbol interference(ISI) and multipath fading are important problems in digital communications. Precoding is one of the techniques used for ISI/multipath cancellation. Conventional precoding techniques, such as Tomlinson-Harashima precoding and trellis preceding, and other ISI cancellation techniques, such as decision feedback equalizers, usually suffer from spectrum-null characteristics in frequency-selective fading channel. Further, conventional precoding methods require knowledge of the ISI channel at the transmitter, i.e., a feedback channel is needed.
An object of the invention is to provide a digital wireless communications system in which a system receiver is capable of recovering information received from a system transmitter without realizing the intersymbol interference channel characteristics received in an antenna array of the system receiver.
A further object of the invention is to reduce the hardware needed in and increase the capacity of a high-speed, low-power digital wireless communications system such as personal communication services (PCS).
A still further object of the present invention is to provide a digital wireless communications system in which the system receiver receives digitally-encoded data at a reduced sampling rate.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention comprises a digital wireless communications system including: a receiver having an antenna array for receiving transmitted digitally-encoded signals; and a transmitter for transmitting the digitally-encoded signals, the transmitter having a plurality of precoders which eliminate intersymbol interference in the transmitted signals such that the receiver recovers digital information from the transmitted signals which is free from intersymbol interference.
The invention further comprises a method for eliminating intersymbol interference in a digital wireless communications system comprising a receiver having an antenna array for receiving transmitted digitally-encoded signals, and a transmitter for transmitting the digitally-encoded signals, the method comprising the step of: providing the transmitter with a plurality of precoders which eliminate intersymbol interference in the transmitted signals such that the receiver recovers digital information from the transmitted signals which is free from intersymbol interference.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.